Drugs alter the expression of many important genes in the brain. These changes in gene expression are likely to contribute to long-term drug effects, such as tolerance and dependence. Continuing studies of the genes encoding endogenous morphine-like peptides suggest that drug-induced gene regulation depends on the circuits connecting to the neuron and the duration of drug exposure. During the past year, the laboratory has monitored cellular levels of neuropeptide mRNAs and of the transcription factor genes that may alter expression of these mRNAs to assess gene regulation related to functional activity in these neurons. In studies completed during this year, transsynaptic regulation of preproenkephalin expression in pain-modulating neurons of the nucleus caudalis of the spinal tract of the trigeminal was described in detail. and the AP-1 and CREB family transcription factors that could participate in this regulation were identified. Transgenic mice possessing a specific portion of the preproenkephalin promoter were produced, and their responsiveness to only certain trans-synaptic activators of preproenkephalin expression noted. Interesting effects of expressed minigene constructs on male fertility and testicular function were found. Finally, approaches to directly modifying these transcription factor pathways in brain with intraparenchymal injection of modified oligonucleotides were validated. These approaches allow understanding of mechanisms important for attempts to therapeutically manipulate drug influences on gene expression. Preliminary results from a human study using opiate antagonists to change the expression of these opioid peptide genes provide evidence for the efficacy of such targeted therapies.